In optical transmission systems it is frequently desired to use an optical fibre for bidirectional communication. This is achieved in most practical systems by using wavelength multiplexing so that transmission in one direction takes place at one or more wavelengths, and so that transmission in the other direction takes place at one or more other wavelengths different from the first-mentioned wavelengths.
Since the signals are transmitted through an optical fibre, they will be subjected to attenuation, which necessitates amplification of the optical signals if they are to be transmitted over great distances.
According to the prior art this bidirectional amplification may be achieved by suitable coupling of wavelength multiplex couplers and a unidirectional amplifier. This method, however, is complicated and consequently involves relatively huge costs.